To effectively drive a permanent magnet synchronous motor (PMSM), the motor control system requires accurate information on rotor position. Sensors such as Hall sensors may be used to sense rotor position, however this increases cost and weight, decreases reliability, and subjects the motor to temperature limitations imposed by the operational limitations of the sensors.
Sensorless control is known, and typically involves estimation of the rotor speed and/or position based on induced EMF or back-EMF occurring in an unenergized main or auxiliary stator winding. One well-known technique involves monitoring zero voltage crossings in the back EMF of the unenergized motor winding, which can be used to establish the position of the rotor, which is then fed back to the commutating circuit to provide proper commutation sequence to the stator windings. Difficulties are encountered, however, due to EMF interference in the winding caused by the driven windings, and filters added to reduce the interference themselves introduce delay and cost. Improvement in sensorless control is therefore desirable, and it is an object of the present invention to provide such improvement.